1. Field of the Invention
The present invention relates to an optical apparatus such as a liquid crystal projector provided with a light emitting section having a concave mirror to be used as a light source and a cooling method thereof. Further, the present invention relates to an optical apparatus provided with a low temperature and high pressure region and a high temperature and low pressure region. Furthermore, the present invention relates to a cooling mechanism for cooling an optical apparatus, especially a liquid crystal projector provided with a high temperature region in which a large amount of heat is generated such as a discharge emission type light source of high output and also provided with a low temperature region which tends to be damaged by heat such as an electric circuit and a light bulb, wherein the high and low temperature regions are arranged substantially adjacent to each other in the apparatus.
2. Description of the Related Art
The liquid crystal projector magnifies an image displayed on a transmission type liquid crystal panel, the width across corners of which is several inches, by 10 to 100 times using an optical system, and the magnified image is projected on a screen. Therefore, a discharge emission type metal halide lamp with an input power of 150 to 200 W is used, from which light is emitted when a metal gas is subjected to an electric discharge. The metal halide lamp includes a light emitting section composed of a silica tube in which a light emitting material such as mercury or a rare earth metal is enclosed, and an electrode is used for discharge. The metal halide lamp is fixed to a concave mirror and assembled to an optical apparatus such as a liquid crystal projector.
The metal halide lamp is disadvantageous in that light emission of mercury gains predominance so that the light emitting efficiency is sharply lowered when the temperature of a silica tube in the light emitting section is not more than 800.degree. C. approximately, and further the light emitting spectrum is suddenly changed, the luminance is lowered and white becomes unbalanced in the display. On the other hand, the following problems are also caused. Depending upon the electric power consumption of the light emitting section, when the temperature of the silica tube in the light emitting section is raised to more than 900.degree. C. approximately, a reaction between the silica tube and light emitting material rapidly occurs, so that the composition of the light emitting material is changed and the spectrum is also changed, and as a result, the displayed colors are changed, and at the same time the silica tube becomes opaque, so that light is absorbed and scattered by the silica tube and the amount of light generation is substantially reduced. Further, the silica tube is expanded and deformed by the high inside pressure (15 to 20 atm), light is absorption by the opaque silica tube and a temperature rise is caused by heat generation. Therefore, it is impossible to use the metal halide lamp over a long period of time.
About 25% of electric power input into the metal halide lamp is converted into visible light, and about 15% of electric power is irradiated, as light, onto a liquid crystal panel, or a dichroic mirror arranged on the optical unit, to be irradiated.
About 10% of the electric power is converted into visible light and about 75% of the electric power that has not been converted into visible light, that is, about 85% of the total electric power is changed into heat in the lamp house.
The larger the amount of electric power input into the metal halide lamp, the higher the luminance of display. In proportion to the increase in the amount of electric power, the amount of generated heat is increased. Therefore, it is important to protect a low temperature region, in which an optical system is accommodated, from a heat flow which flows out from a lamp house in a high temperature region. Conventionally, in order to protect the low temperature region from the heat flow, the following countermeasures are taken: The low temperature region is separate from the high temperature region; a heat insulating plate is inserted between the low and high temperature regions; the amount of generated heat is restricted by using a metal halide lamp into which electric power of not more than 200 W is inputted; and the capacity of the cooling fan for cooling the low temperature region is greatly increased.
However, according to the method in which the low temperature region is separate from the high temperature region, it is necessary that both regions are separate from each other by not less than several tens of mm. Therefore, dimensions of the apparatus are increased, and further the distance to the liquid crystal panel arranged in the low temperature region is increased by several tens mm, so that an amount of light collected onto the liquid crystal panel is reduced, and the luminance of the display is lowered.
Glass fiber cloth or mica is used for a heat insulating plate inserted between the low and high temperature regions. However, the heat insulating plate made of glass fiber cloth or mica is disadvantageous in the dust is caused, the heat insulating plate is expensive and the structure is complicated.
According to the method in which the capacity of the low temperature region cooling fan is greatly increased, a high level of noise is produced, and further the electric power consumption of the apparatus is increased.
For example, in the case of a metal halide lamp, the electric power input of which is 250 W, temperatures of a housing and a steel sheet wall between an optical unit to be irradiated and a lamp house are raised to 100.degree. to 200.degree. C. Due to the heat which flows through the steel sheet wall into the optical unit to be irradiated, the temperature of a liquid crystal panel, especially the temperature of a portion of the liquid crystal panel close to the steel sheet wall is raised by about 10.degree. C. The increase of temperature caused by a heat flow which flows from the lamp house through the steel sheet wall cannot be neglected in the case of a liquid crystal panel, the upper limit of normal operation temperature of which is only 60.degree. C.
In order to solve the above problems, a metal halide lamp of low electric power (not more than 150 W), the silica tube of which is less susceptible to deformation of expansion even in the case of natural air cooling, is conventionally used so that the silica tube can be cooled by an air flow and a circulating current.
Alternatively, an air flow is introduced into the concave mirror from a lower cutout portion or a side cutout portion of the concave mirror so that the silica tube can be cooled, and a hot air flow is forcibly discharged outside from an upper cutout portion of the concave mirror.
In order to operate the liquid crystal panel in a normal condition, it is required that the temperature of the liquid crystal panel which is arranged in front of the light source at a position distant from the light source only by 100 to 250 mm is maintained at not more than 60.degree. C. Also, it is required that the temperature of a polarized light film which is arranged on both sides of the liquid crystal panel for absorbing unnecessary light by converting it into heat is maintained to be not more than 60.degree. C.
Further, a printed board to which an electric circuit for driving the liquid crystal panel is assembled must be arranged close to the liquid crystal panel, has an upper limit of allowable operation temperature of 60.degree. C., and has an electric power consumption of not more than 10 W. In the case of a printed board onto which an electric power circuit is assembled for supplying DC power, the electric power consumption is about 30 W, however, the upper limit of allowable operation temperature is 60.degree. C.
In the case of a printed board onto which an electric circuit for turning on a light source to send light to the pretreatment system is assembled, it is necessary that the printed board is arranged close to the light source of high temperature, and further the electric power consumption is about 50 W, however, the upper limit of allowable operation temperature is only 70.degree. C.
In order to maintain the image quality, the positional relationship, the distance and direction of the optical system including the light source, the pretreatment system, the after-treatment system for the liquid crystal panel and projection lens, must be maintained in a predetermined condition. Accordingly, the degree of freedom of designing the optical system is very low with respect to the layout in the apparatus.
Consequently, it is important to effectively transport the heat generated by the optical system, the electric circuits and other members and discharge it outside of the apparatus so that the temperatures of all the members in the apparatus can be maintained appropriate for normal operation.
Conventionally, the outside air sucked through a suction port provided close to the liquid crystal panel is first blown against the liquid crystal panel. Next, the air is discharged outside of the apparatus through an exhaust port arranged close to the light source approximately straight. The printed board and light source are arranged approximately between the suction and exhaust ports, wherein the printed board is arranged upstream, and the light source is arranged in the downstream, in the air flow.